Synchronization control device for servo motors

ABSTRACT

Position droops produced in main and auxiliary servo motors when main and auxiliary spindles are coupled to each other through a workpiece are obtained by adders, respectively, a difference between those position droops is obtained by a comparator before coupling, a divider divides the position droop difference by the speed of the auxiliary servo motor at the time of detecting the position droop to obtain a position droop difference per a unit speed. Then, after both the spindles are coupled to each other, a multiplier multiplies the position droop difference by the present speed of the auxiliary servo motor, and the adder adds that value to the position command to the auxiliary servo motor. With the above structure, even if coupling is made through the workpiece and synchronization drive is made, an excessive torque can be prevented from occurring, an influence of a change of the change gear ratio with a time is eliminated and a displacement occurring at the time of coupling is corrected.

TECHNICAL FIELD

The present invention mainly relates to a spindle synchronization drivefor a machine tool, and to a synchronization control device for a servomotor which is so designed as to drive a main spindle and an auxiliaryspindle which are coupled to and decoupled from each other through aworkpiece by a main servo motor and a sub-servo motor, respectively.

BACKGROUND ART

Up to now, a large number of inventions have been made on a spindlesynchronization control for a machine tool, for example, as disclosed inJapanese Patent Unexamined Publication No. Hei 1-228750, Japanese PatentUnexamined Publication No. Hei 1-228751, Japanese Patent UnexaminedPublication No. Hei 1-228752, Japanese Patent Unexamined Publication No.Hei 2-109605, etc.

All that are disclosed in those publications are inventions for makingaccurate synchronization by driving those two main spindles by a speedcontrol servo motor when two spindles are decoupled from each other notthrough a workpiece.

In particular, the invention disclosed in Japanese Patent UnexaminedPublication No. Hei 1-228750 discloses a technique by which a speeddeviation between both the spindles is detected to make the speedsynchronization of those two spindles. The invention disclosed inJapanese Patent Unexamined Publication No. Hei 1-228751 discloses atechnique by which synchronization is made additionally using apositional deviation between both the spindles. The invention disclosedin Japanese Patent Unexamined Publication No. Hei 1-228752 discloses atechnique by which a torque is employed in addition to the speed and theposition. The invention disclosed in Japanese Patent UnexaminedPublication No. Hei 2-109605 discloses a technique by which not only thespeed of the spindles but also the position thereof is synchronized.Thus, those inventions are designed such that the spindles are driven bythe speed control servo motor.

Also, the invention disclosed in Japanese Patent Unexamined PublicationNo. Hei 2-41693 conducts the positional control by an auxiliary spindledrive servo motor out of two spindle servo motors. However, in theinvention, a command is of a position signal from a main spindleposition detector, which clearly causes a lag, and both the main andauxiliary spindles are not driven by the same position command.

The reason that most of the prior art drive the spindles by the speedcontrol servo motor as described above is that in the spindle driveservo motor for a machine tool, since there are many cases in which arated speed is high although the maximum torque is not so large, if themechanical coupling of both the spindles is made during a period of timeuntil the speed reaches the rated speed since the servo motor starts, orafter the speed is greatly changed, the speed control that enables atorque required for increasing or decreasing the speed to be increasedor decreased by the maximum torque generated by the motor isadvantageous in time.

However, what cannot accurately adjust the change gear ratio between theservo motor and the spindles is required to conduct the control of thespindle per se which serves as a reference, that is, the positionalcontrol of the spindle which is a final mechanical edge.

However, the above-described conventional synchronization control devicefor a servo motor suffers from problems stated below.

When the main spindle and the auxiliary spindle are coupled to eachother through a workpiece both ends of which are gripped by chucks, ifthere is a difference in the total position gain between both thespindles, that is, the position gain pertaining to the operation of thespindles and the change gear ratio between the spindles and the servomotors (the rotating speed of the servo motors/the rotating speed of thespindles), a difference occurs between the position droop of the mainand auxiliary spindles. This causes such problems that the torque of oneservo motor reaches the torque limit, or if the fastening force of thechucks is weak, the workpiece is displaced and damaged.

In other words, the difference in total gain is caused because anaccurate change gear ratio is not obtained in the case where a flexiblestructure such as a belt is employed for torque transmission between thespindles and the servo motors. Then, if there is a slight differencebetween the actual change gear ratio and a parameter set on amplifiersof the servo motors, a difference occurs in the above position droop inthe position control. On the other hand, a speed difference occurs incase of the speed control.

For the above reason, in any cases, a phase difference occurs within themain and auxiliary servo motors which are coupled to each other, as theresult of which the load torque of the servo motors is increased up tothe torque limit value, or the workpiece is displaced if the fasteningforce of the chucks is weak, to thereby damage the workpiece.

In particular, as the representative application method of the spindlesynchronization, a work for supporting both ends of the workpiece andcutting a center portion of the workpiece is greatly effective inacceleration of processes after cutting, and since the cutting work isconducted at a constant peripheral speed for the workpiece, the spindlesynchronization control that enables the rapid acceleration/decelerationis desirable in a reduction of working time. However, the prior art doesnot permit even a slight displacement of the above change gear ratio.

Also, in general, since there are many cases in which the chuck of theauxiliary spindle grips the workpiece in a state where the main spindlepermits the workpiece to rotate, a large force is exerted on the mainand auxiliary spindles and the servo motors at that time, the rotationsof the main and auxiliary spindles are lowered, resulting in the eventthat mechanical coupling is made in a state where the main and auxiliaryspindles are displaced from a designed position. In this event, thecoupling work of the auxiliary spindle must be made again, therebyleading to such a problem that troublesomeness or time is taken forworking.

FIGS. 16 and 17 show a change in a difference of the position droopbefore and after both of the main and auxiliary spindles are coupled toeach other through the workpiece, and a change in a difference of theposition droop before and after both of the main and auxiliary spindlesare coupled to each other not through the workpiece.

FIG. 17 showing a change in the difference of the position droop whenthere is no workpiece shows an appearance in which when a work offastening the chuck on the auxiliary spindle side is made, an extremelylarge force is exerted on the servo motor on the auxiliary spindle side,with the results that the speed is decreased and the difference of theposition droop becomes temporarily large but becomes soon small.

However, in the case where the workpiece is held by the chuck on themain spindle side and a work of fastening the chuck on the auxiliaryspindle side is made, coupling may be made at the time when the speeddecreases. If the chuck on the auxiliary spindle side is fastened tocouple the main and auxiliary spindles to each other in the abovemanner, the difference between the position droops of the main andauxiliary spindles rises suddenly in a stepped state as shown in FIG. 16and is continued until the position droop is released till chuck-offsince chuck-on.

For that reason, since in the position control, a force that recovers aphase lag is exerted and reaches the torque limit value, it is necessaryto correct the lag amount and to try coupling again.

In view of the above, the present invention has been made to solve theabove problems, and therefore has an object to provide a synchronizationcontrol device for a servo motor, which enables the accuratesynchronization driving of a main spindle and an auxiliary spindle andenables a rapid acceleration/deceleration driving even if a differenceexists between the position droops of both the spindles when the mainspindle and the auxiliary spindle are coupled to each other, and inparticular, which can automatically cope with a change in a change gearratio with a time even if a flexible structure such as a belt isemployed for torque transmission between the spindles and servo motors.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, a synchronization control devicefor a servo motor according to the present invention comprises: a mainspindle and an auxiliary spindle each having a chuck that holds bothends of a workpiece and being coupled to or decoupled from each otherthrough the workpiece; a main servo motor and an auxiliary servo motorwhich rotationally drive the main spindle and the auxiliary spindle,respectively; a main spindle position detector and an auxiliary spindleposition detector which are fitted to the main spindle and the auxiliaryspindle, respectively, and output a position feedback signal; a mainservo motor speed detector and an auxiliary servo motor speed detectorwhich are fitted to the main servo motor and the auxiliary servo motor,respectively, and output a speed feedback signal; a main servo amplifierand an auxiliary servo amplifier which drive the main servo motor andthe auxiliary servo motor, respectively; and a numerical control unitwhich outputs a speed command and a position command to the main servoamplifier and the auxiliary servo amplifier, wherein each of the mainservo amplifier and the auxiliary servo amplifier includes: a positioncontrol section which outputs a position droop and the speed commandresponsive to the position droop on the basis of inputs of the positionfeedback signal from the position detector and the position command fromthe numerical control unit; a speed control section which outputs acurrent command on the basis of the speed command from the positioncontrol section and the speed feedback signal from the speed detector;and a current control section which controls a supply current to themotor on the basis of the current command from the speed control sectionand the current feedback signal which flows in the motor, where in theauxiliary servo amplifier further includes a position correction sectionwhich obtains position correction data based on a difference in positiondroop between the main spindle position droop and the auxiliary spindleposition droop and the speed feedback signal from the speed detector ofthe auxiliary servo motor when the main servo motor and the auxiliaryservo motor rotate in a state where the main spindle and the auxiliaryspindle are decoupled from each other, and wherein the position controlsection of the auxiliary servo amplifier adds the position correctiondata from the position correction section to the position droop andoutputs the speed command responsive to the position droop obtained byaddition when the main spindle and the auxiliary spindle are coupled toeach other through the workpiece, and the main servo motor and theauxiliary servo motor rotate in synchronization with each other.

Also, there is characterized in that the position correction sectionincludes: a comparing means for obtaining a position droop differencebetween the main spindle position droop and the auxiliary spindleposition droop when the main servo motor and the auxiliary servo motorrotate in a state where the main spindle and the auxiliary spindle aredecoupled from each other; a dividing means for dividing the positiondroop difference from the comparing means by the speed feedback signalfrom the speed detector of the auxiliary servo motor; a storing meansfor storing a division value from the dividing means; a multiplyingmeans for multiplying the division value stored in the memory means bythe speed feedback signal from the speed detector of the auxiliary servomotor; and a switch means for switching so as to output, as the positioncorrection data which are outputted to the position control section ofthe auxiliary servo amplifier, the position droop difference from thecomparing means when the main servo motor and the auxiliary servo motorrotate in a state where the main spindle and the auxiliary spindle aredecoupled from each other, and to output, as the position correctiondata which are outputted to the position control section of theauxiliary servo amplifier, a multiplication value from the multiplyingmeans when the main spindle and the auxiliary spindle are coupled toeach other through the workpiece, and the main servo motor and theauxiliary servo motor rotate.

Further, there is characterized in that the position correction sectionincludes: a position droop ratio calculating means for calculating theratio of the position droop of the main spindle to the position droop ofthe auxiliary spindle when the main servo motor and the auxiliary servomotor rotate in a state where the main spindle and the auxiliary spindleare decoupled from each other; and a storing means for storing theposition droop ratio from the position droop ratio calculating means,and the position control section includes: a position droop calculatingmeans for calculating the position droop of the auxiliary spindle on thebasis of inputs of the position feedback signal from the positiondetector and the position command from the numerical control unit; aposition gain multiplying means for multiplying the position droop fromthe position droop calculating means by a position gain of the auxiliaryservo motor which is stored in advance; a change gear ratio multiplyingmeans for multiplying a multiplication output from the position gainmultiplying means by a change gear ratio of the auxiliary servo motorwhich is stored in advance; and a change gear ratio correcting means forcorrecting the change gear ratio by multiplying the change gear ratiowhich is stored in the change gear ratio multiplying means in advance bythe position droop ratio which is stored in the memory means when themain spindle and the auxiliary spindle are coupled to each other throughthe workpiece, and the main servo motor and the auxiliary servo motorrotate in synchronization with each other.

Still further, there is characterized in that the change gear ratiocorrecting means includes a filter which subjects the position droopratio stored in the storing means to a first-order lag processing.

Yet still further, there is characterized in that said positioncorrection section includes: a position droop ratio calculating meansfor calculating the ratio of the position droop of said main spindle tothe position droop of said auxiliary spindle when said main servo motorand said auxiliary servo motor rotate in a state where said main spindleand said auxiliary spindle are decoupled from each other; and a storingmeans for storing the position droop ratio from the position droop ratiocalculating means, and the position control section includes: a positiondroop calculating means for calculating the position droop of theauxiliary spindle on the basis of inputs of the position feedback signalfrom the position detector and the position command from the numericalcontrol unit; a position gain multiplying means for multiplying theposition droop from the position droop calculating means by a positiongain of the auxiliary servo motor which is stored in advance; a changegear ratio multiplying means for multiplying a multiplication outputfrom the position gain multiplying means by a change gear ratio of theauxiliary servo motor which is stored in advance; and a position gaincorrecting means for correcting the position gain by multiplying theposition gain which is stored in the position gain multiplying means inadvance by the position droop ratio which is stored in the storing meanswhen the main spindle and the auxiliary spindle are coupled to eachother through the workpiece, and the main servo motor and the auxiliaryservo motor rotate in synchronization with each other.

Yet still further, there is characterized in that the position gaincorrecting means includes a filter which subjects the position droopratio stored in the storing means to a first-order lag processing.

Yet still further, there is characterized in that the positioncorrection section stores, in the storing means, a division valueobtained from dividing the position droop difference from the comparingmeans by the speed feedback signal from the speed detector of theauxiliary servo motor when the main servo motor and the auxiliary servomotor rotate by a predetermined r.p.m. or higher in a state where themain spindle and the auxiliary spindle are decoupled from each other.

Yet still further, there is characterized in that the positioncorrection section stores in the storing means the position droop ratiofrom the position droop ratio calculating means when the main servomotor and the auxiliary servo motor rotate by a predetermined r.p.m. orhigher in a state where the main spindle and the auxiliary spindle aredecoupled from each other.

Yet still further, there is characterized in that the positioncorrection section further includes: a switch disposed between themultiplying means and the memory means and being closed when the mainservo motor and the auxiliary servo motor rotate in a state where themain spindle and the auxiliary spindle are decoupled from each other andopened when the main spindle and the auxiliary spindle are coupled toeach other through the workpiece, and the main servo motor and theauxiliary servo motor rotate; a subtractor which subtracts, from theposition droop from the comparing means, the position droop due to themultiplication value from the multiplying means that multiplies thedivision value stored in the storing means through the switch by thespeed feedback signal from the speed detector of the auxiliary servomotor when the main spindle and the auxiliary spindle are coupled toeach other through the workpiece, and the main servo motor and theauxiliary servo motor rotate; and a coupling-time displacementcorrection section which outputs a difference in the position droopbetween the main spindle and the auxiliary spindle which is obtainedfrom the subtractor and produced before and after the main spindle andthe auxiliary spindle are coupled to each other through the workpiece tothe position control section as the position correction data.

Yet still further, there is characterized in that the coupling-timedisplacement correction section further includes: zero data generatingmeans for generating zero data as the position correction data; and anaccumulating means for accumulating the position correction data fromthe subtractor which is produced every time the main spindle and theauxiliary spindle are coupled to each other through the workpiece,wherein when the coupling-time displacement correction section outputsthe zero data from the zero data generating means or the positioncorrection data from the accumulating means as the position correctiondata which are outputted to the position control section immediatelyafter coupling when the main spindle and the auxiliary spindle whichhave been coupled to each other through the workpiece are decoupled fromeach other.

Yet still further, there is characterized in that there are provided acomparator which compares the speed command from the position controlsection with the speed command from the numerical control section, and acommand switching means for outputting the speed command from theposition control section to the speed control section to conductposition control when a comparison difference is within a given error,and for outputting the speed command from the numerical control sectionto the speed control section to conduct position control when thecomparison difference is not within the given error.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram showing a synchronization control devicefor a servo motor in accordance with Embodiment 1 of the presentinvention.

FIG. 2 is a structural diagram showing the structure of a main servoamplifier 16 and an auxiliary servo amplifier 26 in FIG. 1.

FIG. 3 is a structural diagram showing a modified example of the mainservo amplifier 16 and the auxiliary servo amplifier 26 in FIG. 1.

FIG. 4 is a structural diagram showing of a coupling-time displacementcorrection section 35 shown in FIG. 2 in detail.

FIG. 5 is a structural diagram showing a modified example of thecoupling-time displacement correction section 35 shown in FIG. 2 indetail.

FIG. 6 is a state explanatory diagram showing a state of the respectiveswitches (SW) at the time of chuck-off and a state of the respectiveswitches at the time of chuck-on in the case of conducting a correctionprocessing where a value of a final droop difference is divided by afinal speed and then multiplied by a present speed, immediately beforechuck-on.

FIG. 7 is a flowchart showing a correction processing where a finaldroop value is divided by the final speed and then multiplied by thepresent speed at the time of chuck-on.

FIG. 8 is a state explanatory diagram showing a state of the respectiveSWs in the case of conducting a correction processing where a ratio ofthe position droop of an auxiliary servo motor to the position droop ofa main servo motor is multiplied by a change gear ratio of the auxiliaryservo motor.

FIG. 9 is a flowchart showing a state of the switches shown in FIG. 8and a correction processing where the ratio of the position droop of theauxiliary servo motor to the position droop of the main servo motorshown in FIG. 3 is multiplied by the change gear ratio of the auxiliaryservo motor or a position loop gain.

FIG. 10 is a state explanatory diagram showing a state of the respectiveSWs in the case of conducting a correction processing where a differencevalue in the droop difference between the main servo motor and theauxiliary servo motor at the time of chuck-on and chuck-off is added tothe position command of the auxiliary servo motor.

FIG. 11 is a flowchart showing a state of the switches shown in FIG. 10and a correction processing where a difference value in the droopdifference between the main servo motor and the auxiliary servo motor atthe time of chuck-on and chuck-off are added to the position command ofthe auxiliary servo motor.

FIG. 12 is a state explanatory diagram showing a state of the respectiveSWs in the case of conducting a processing of selecting a function ofsetting a correction added to the position command of the auxiliaryspindle servo motor to 0 immediately after coupling, and a function ofaccumulating a displacement produced in each coupling to correct theposition command of the auxiliary spindle servo motor.

FIG. 13 is a flowchart showing a state of the respective switches shownin FIG. 12 and a correction processing of selecting a function ofsetting a correction added to the position command of the auxiliaryspindle servo motor to 0 immediately after coupling, and a function ofaccumulating a displacement produced in each coupling to correct theposition command of the auxiliary spindle servo motor without returningthe position to a position immediately before coupling, when the mainand auxiliary servo motors which have been coupled to each other throughthe workpiece shown in FIG. 5 are decoupled from each other.

FIG. 14 is a state explanatory diagram showing a state of the respectiveSWs in the case of conducting a processing that can change the speedcontrol to the position control and vice versa when the speed command iswithin a predetermined error.

FIG. 15 is an explanatory diagram showing the speeds of the main servomotor and the auxiliary servo motor, and a switching state between thespeed control and the position control due to the processing shown inFIG. 11.

FIG. 16 is an explanatory diagram showing a change in a difference ofthe position droop before and after coupling when the main and auxiliaryspindles are coupled to each other through the workpiece in the priorart.

FIG. 17 is an explanatory diagram showing a change in the difference ofthe position droop in the case where there is no workpiece as comparedwith FIG. 16.

BEST MODES OF CARRYING OUT THE INVENTION EMBODIMENT 1

FIG. 1 is a structural diagram showing a synchronization control devicefor a servo motor in accordance with Embodiment 1 of the presentinvention.

Referring to FIG. 1, reference numeral 10 denotes a main spindle, and 14is a main servo motor that rotates the main spindle 10, where the mainspindle 10 and the main servo motor 14 are coupled to each other througha belt 12. Also, reference numeral 20 denotes an auxiliary spindle and24 is an auxiliary servo motor which are coupled to each other through abelt 22.

Reference numeral 1 denotes a workpiece which is held by a chuck 11fitted to the main spindle 10 and a chuck 21 fitted to the auxiliaryspindle 20. It is needless to say that coupling between the main spindle10 and the main servo motor 14 and between the auxiliary spindle 20 andthe auxiliary servo motor 24 may be made by not the belts 12 and 22 butnormal gears.

The main spindle 10 is fitted with a position detector 15 for positionfeedback. Likewise, the auxiliary spindle 20 is fitted with a positiondetector 25 for position feedback.

The main servo motor 14 is fitted with a speed detector 17 for speedfeedback. Likewise, the auxiliary servo motor 27 is fitted with a speeddetector 27 for speed feedback.

Reference numeral 16 denotes a main servo amplifier that drives the mainservo motor 14, and 26 is an auxiliary servo amplifier that drives theauxiliary servo motor 26. In this example, each of the main servoamplifier 16 and the auxiliary servo amplifier 26 is made up of a speedcontrol circuit having a microprocessor, a memory and so on, a positioncontrol circuit and a power drive circuit having a power transistor andso on.

Reference numeral 2 denotes a numerical control unit (CNC), and 3 is amain spindle speed command producer that produces a speed command Wr1*which is commanded to the main servo amplifier 16 within the numericalcontrol unit (CNC) 2. Reference numeral 5 denotes a main positioncommand producing circuit that converts the speed command signal Wr1*outputted from the main spindle speed command producer 3 into a positioncommand θ1* which is commanded to the main servo amplifier 16.

Also, the main position command producing circuit 5 is so structured asto produce an output signal by also using a position feedback θ1outputted from the position detector 15 which is fitted to the mainspindle 10 and turns on a switch SWA1 to validate an output to the mainservo amplifier 16 when switching the speed control to the positioncontrol.

Likewise, reference numeral 4 denotes an auxiliary spindle speed commandproducer that produces a speed command Wr2* which is commanded to theauxiliary servo amplifier 26, and 6 is an auxiliary position commandproducing circuit that converts the speed command signal Wr2* into aposition command θ2*. Also, the auxiliary position command producingcircuit 6 turns on a switch SWA2 to validate an output to the auxiliaryservo amplifier 26 when switching a speed command to a position command.Further, each of the main servo amplifier 16 and the auxiliary servoamplifier 26 has a function of receiving position droops (θ1*−θ1,θ2*−θ2) of the counter servo amplifier.

FIG. 2 shows a specific structure of the main servo amplifier 16 and theauxiliary servo amplifier 26.

Although the main servo amplifier 16 and the auxiliary servo amplifier26 are not different in structure from each other, FIG. 2 mainly showsthe structure of the auxiliary servo amplifier 26 and represents thestructure of the main servo amplifier 16 at the minimum.

Referring to FIG. 2, a description will be mainly given of the structurewithin the auxiliary servo amplifier 26. The auxiliary servo amplifier26 includes a position control section 39, a speed control section 31, acurrent control section 34, a comparator 36 and a position correctionsection 40. It should be noted that the main servo amplifier 16 has noposition correction section 40.

First, the structure within the position control section 39 will bedescribed. Reference numeral 38 denotes an adder that adds the positioncommand θ2* outputted from the numerical control unit (CNC) 2 shown inFIG. 1, a signal a that has passed through a switch SW1 and a filter 29within the position correction section 40 and a signal β outputted fromthe coupling-time displacement corrector 35 within the positioncorrection section 40 together and subtracts the position feedbacksignal θ2 outputted from the position detector 25 therefrom.

Also, reference numeral 28 denotes a position gain multiplier thatmultiplies an output signal from the adder 38 by a position gain Kp, and29 is a change gear ratio multiplier that multiplies an output of theposition gain multiplier 28 by a change gear ratio KG which is stored inadvance, where the output from the change gear ratio multiplier 29 isoutputted as a speed command.

The comparator 36 compares the speed command Wr2* outputted from thenumerical control unit (CNC) 2 with an output signal from the changegear ratio multiplier 29 and controls a switch SW4 so as to output thespeed command from the position control section 39 to the speed controlsection 31 for conducting position control when a comparison differenceis within a given error, and to output the speed command from thenumerical control section 2 to the speed control section 31 forconducting speed control when the comparison difference is not withinthe given error. The switch SW4 is so designed as to switch the speedcommand inputted to the speed control section 31 according to a commandof the comparator 36 which is based on an output signal from the changegear ratio multiplier 29 of the position control section 39 and thespeed command Wr2* from the numerical control unit (CNC) 2.

The speed control section 31 is so designed as to subtract the speedcommand of its input signal from the speed feedback signal Wr2 from thespeed detector 27 fitted to the auxiliary servo motor 24 and converts asubtracted value into a current command and output the current command.

The current control section 34 is so designed as to subtract the abovecurrent command from the feedback signal of a current that flows in theauxiliary servo motor 24 and processes the signal so that the auxiliaryservo motor 24 is driven according to the current command, and to outputthe processed signal to the auxiliary servo motor 24. As shown in thefigure, the current control section 34 includes an adder 50, a currentgain section 32, a switch SWB, a limiter 51 and a current amplificationsection 33.

The position correction section 40 includes a function of producingthree kinds of data consisting of data a with respect to first andsecond kinds of methods for correcting inaccuracy of the change gearratio and a second kind of data β which are a correction of lag causedat the time of mechanical coupling. The second kind of data β which arecaused at the time of mechanical coupling is produced by thecoupling-time displacement corrector 35.

Specifically, in the structure within the position correction section40, reference numeral 7 denotes a comparator that arithmeticallyoperates a difference between the data of the position droop Dr1(=θ1*−θ1) of the main servo amplifier 16 and the position droop data Dr2(an out put of the adder 38) of the auxiliary servo amplifier 26. Also,reference numeral 9 denotes a divider which divides the above differencein the position droop which is an output signal from the comparator 7 bythe present motor speed of the auxiliary servo motor 24 which isdetected by the speed detector (PG) 27 of the auxiliary servo motor 24.

Reference numeral 18 denotes a memory which stores an output result ofthe divider 9 through a switch SW3 therein, and 19 is a multiplier thatmultiplies the value of the memory 18 by the present motor speed. Theoutput data of the multiplier 19 pass through a switch SW1-b and afilter 29 and are outputted to the adder 38 and the coupling-timedisplacement corrector 35 as the data α. The comparator 7 arithmeticallyoperates a difference in the position droop between the position droopof the coupled main servo motor 14 and itself (auxiliary servo motor 24)and transmits the difference to the adder 38 through the switch SW1 andthe filter 29 to correct the position command of the auxiliary servomotor 24.

However, the correcting method is effective when the main and auxiliaryservo motors 14 and 24 are decoupled from each other, but ineffectivewhen they are coupled to each other through the workpiece 1 and conductacceleration/deceleration control because of lag. In this case, aposition displacement may occur to the degree that the torque rises upto a torque limit value. Accordingly, a desired control can be made byusing the difference between both the position droops which is obtainedbefore coupling in a feed-forward manner, that is, as a predeterminedconstant.

Reference numeral 35 denotes a coupling-time displacement corrector thatcompares the output data of the comparator 7 with data from themultiplier 19 through the switch SW1-b in a subtraction manner. Theoutput signal from the coupling-time displacement corrector 35 isoutputted to the adder 38 through an internal filter as the data β.

FIG. 3 shows a modified example of the auxiliary servo amplifier 26shown in FIG. 2 where the structures within the position correctionsection 40 and the position control section 39 are different from thosein FIG. 2.

Referring to FIG. 3, the same parts as the structures shown in FIG. 2are designated by identical references, their descriptions will beomitted and only new references will be described.

In the structure of the position correction section 40, reference 9 adenotes a divider that divides the data of the position droop Dr2(=θ2*−θ2) of the auxiliary servo amplifier which are outputs of an adder38 a by the data of the position droop Dr1 (=θ1*−θ1) of the main servoamplifier 16 which are outputs from an adder 48, and 18 a is a memorythat inputs and stores its result through the switch SW3 therein.

In the structure of the position control section 39, reference numeral30 and 30 a denote a change gear ratio correction section that correctsa value of the change gear ratio KG of the change gear ratio multiplier29 and a position gain correction section that corrects a value of theposition gain KP of the position gain multiplier 28, on the basis of asignal from the memory 18 a within the position correction section 40,respectively. The change gear ratio correction section 30 and theposition gain correction section 30 a have a filter that conducts afirst-order lag processing inside thereof, respectively. Referencenumeral 38 a denotes an adder that obtains the data of the positiondroop Dr2 (=θ2*−θ2) of the auxiliary servo amplifier.

Subsequently, FIG. 4 shows of the coupling-time displacement correctionsection 35 shown in FIG. 2 in detail.

Referring to FIG. 4, reference numeral 42 denotes an adder whichsubtracts an output of a difference in the position droop between themain servo amplifier 16 and the auxiliary servo amplifier 26 which isthe output data from the comparator 7 from the data outputted from themultiplier 19 through the switch SW1, and 37 is a memory which storesthe output data of the adder 42 through switches SW5 b and SW6 atherein. Reference numeral 41 denotes a filter that varies a change inthe data stored in the memory 37 in a first-order manner, which isoutputted to the adder 38 shown in FIG. 2.

Also, FIG. 5 shows a modified example of the coupling-time displacementcorrection section 35 shown in FIG. 2, which is different from that ofFIG. 4. A circuit in which switches SW5 a, SW6 b and SW6 c are normallyoff in FIG. 5 is directed to that of FIG. 4.

Referring to FIG. 5, when the stored contents in the memory 37 arecleared to 0, 0 is stored in the memory 37 through the switches SW5 aand Sw6 a by a zero setting section 45. Also, reference numeral 43denotes an adder that adds the output value from the adder 42 to theoutput data of the memory 44 through the switches SW5 b and SW6 b, andthe memory 44 stores its result therein.

In this example, the adder 43 and the memory 44 function as anintegrator, and the output data of the integrator are stored in thememory 37 when the switch SW6 c is on (in this situation, the switch SW6a is off).

Then, the operation of this Embodiment 1 will be described.

The operation of the Embodiment 1 includes the following six kinds ofprocessings.

(1) A correction processing where a value of a final droop difference isdivided by a final speed and then multiplied by a present speedimmediately before chuck-on.

(2) A correction processing where the ratio of the position droop of theauxiliary servo motor to the position droop of the main servo motor ismultiplied by the change gear ratio of the auxiliary servo motor or aposition loop gain.

(3) A processing where a value obtained by dividing the value of thefinal droop difference by the final speed in item (1) is stored by apredetermined r.p.m. or higher.

(4) A correction processing where a difference value in the droopdifference between the main servo motor and the auxiliary servo motor atthe time of chuck-on and chuck-off is added to the position command ofthe auxiliary servo motor.

(5) A processing of selecting a function of setting a correction addedto the position command of the auxiliary spindle servo motor to 0immediately after coupling, and a function of accumulating adisplacement produced in each coupling to correct the position commandof the auxiliary spindle servo motor without returning the position to aposition immediately before coupling, when the two main and auxiliaryservo motors which have been coupled to each other through the workpieceare decoupled from each other.

(6) A processing that can change the speed control to the positioncontrol and vice versa when the speed command is within a predeterminederror.

Therefore, each of the processings will be described below.

(1) A correction processing where a value of a final droop difference isdivided by a final speed and then multiplied by a present speedimmediately before chuck-on.

FIG. 6 shows a state of the respective switches (SW) at the time ofchuck-off (refer to (a)) and a state of the respective switches at thetime of chuck-on (refer to (b)) in the case of conducting thisprocessing. In the figure, “O” represents a switch-on state, and “X”represents a switch-off state.

FIG. 7 shows the contents of the correction processing (1) where thevalue of the difference in the final droop is divided by the final speedand then multiplied by the present speed.

In this processing, both of the main and auxiliary servo motors 14 and24 turn on the position control, and first of all, the main servoamplifier 16 arithmetically operates the droop Dr1 (=θ1*−θ1) of the mainspindle 10 (step 100), and the auxiliary servo amplifier 26arithmetically operates the droop Dr2 (=θ2*+α+β−θ2) of the auxiliaryspindle 20 (step 110). Where α=β=0 is set as an initial value.

In this situation, the switches SWA1, SWA2, SW1-a, SW3, SW4 and SW5-bare on, and all of the switches SW1-b, SW2 a, SW5 a, SW6 a, SW6 b andSW6 c are off.

Then, the comparator 7 arithmetically operates a difference β1(=Dr1−Dr2) bet ween the droop Dr1 of the main spindle 10 and the droopDr2 of the auxiliary spindle 20 (step 120). If θ2*=θ1*, β1 (=Dr1−Dr2)becomes (θ2*−θ2)−(θ1*−θ1)=θ1−θ2.

Then, the divider 9 divides the difference β1 in the position droopbetween both the spindles which is obtained in the previous step 120,that is, the position error (θ1−θ2) of both the spindles by the presentrotating speed Wr2 of the auxiliary spindle servo motor 24 which isdetected by the speed detector (PG) 27, thus obtaining a difference Δβ1(=β1/Wr2) per a unit speed (step 130).

Then, the difference Δβ1 per a unit speed is stored in the memory 18through the switch SW3 (step 140), and the multiplier 19 multiplies thedifference Δβ1 thus stored by the present rotating speed Wr2 n of theauxiliary spindle servo motor 24 which is detected by the speed detector(PG) 27, to thereby obtain a corrected value β2 (=Δβ1×Wr2 n) (step 150).In the case where the auxiliary servo motor 24 rotates at a constantspeed, if the switch SW3 is on, β1 =β2 is satisfied because of Wr2=Wr2n, whereas if the switch SW3 is off, Wr2≠Wr2 n is satisfied and β1≠β2 issatisfied because Δβ1 is a constant value.

Subsequently, it is judged whether the chucks 11 and 21 of both thespindles 10 and 20 turn on and both the spindles 11 and 21 are connectedto each other through the workpiece 1, or not (step 160). If the chucks11 and 21 are off (“no” in step 160), since the switch SW1-a is on andthe SW1-b is kept off, the corrected value β which is inputted to theadder 38 becomes β1 obtained by the comparator 7 in the above step 120(step 170).

On the other hand, if the chucks 11 and 21 are on (“yes” in step 160),since the switch SW3 is turned off, the SW1 a is turned off, the SW1 bis turned on and the SW2 b is kept on, the corrected value β which isinputted to the adder 38 becomes β2 obtained by multiplying thedifference Δβ1 stored in the memory 18 by the present rotating speed Wr2n of the auxiliary spindle servo motor 24 which is detected by the speeddetector (PG) 27 through the multiplier 19 (step 180).

The more details will be described. Referring to FIG. 2, the adder 38 ofthe position control section 39 subjects the position command θ2* to theauxiliary spindle 20, the position information of the auxiliary spindle20 from the detector 25 and two kinds of data which correct a differencein position between the main spindle 10 and the auxiliary spindle 20 toaddition and subtraction to constitute a position loop. In this example,two kinds of data is directed to a first kind of data a that correct theinaccuracy of the change gear ratio and a second kind of data β thatcorrect a position lag due to a load caused at the time of mechanicalcoupling.

When the measurement conditions of data are satisfied, and when theconditions are not satisfied by the switch that turns on, the memory 18is not rewritten. If the main and auxiliary spindles 10 and 20 arecoupled to each other, a product of the data stored in the memory 18 andthe speed of the auxiliary servo motor 24 is arithmetically operated bythe multiplier 19 without using the data from the comparator 7. Theoperated result is outputted through the filter 29 and added to theposition command for conducting correction. The first method of thefirst kind in the processing (1) always requires the arithmeticoperation of the corrected data during speed changing.

Therefore, in the processing (1) of the synchronization control devicein accordance with this embodiment 1, a difference in the position droopbetween both the servo motors 14 and 24 when the main spindle 10 and theauxiliary spindle 20 are coupled to each other through the workpiece 1is grasped before coupling. Then, after both the spindles 10 and 20 arecoupled to each other, the corrected value α2 obtained by multiplyingthe difference in the position droop per a unit speed of the auxiliaryservo motor 24 before coupling by the present speed of the auxiliaryservo motor 24 in a feed-forward manner, that is, in a manner the dataare installed in arithmetic operation in advance without any lag on thebasis of the respective position droops of the main servo motor 14 andthe auxiliary servo motor 24 which are grasped before coupling is addedto the position command of the auxiliary servo motor 24 to conductcorrection. As a result, even in the acceleration/deceleration state,the complete synchronization control can be conducted.

(2) A correction processing where the ratio of the position droop of theauxiliary servo motor to the position droop of the main servo motor ismultiplied by the change gear ratio of the auxiliary servo motor or aposition loop gain.

FIG. 8 shows a state of the respective SWs in the case of conducting acorrection processing (2) where a ratio of the position droop of theauxiliary servo motor to the position droop of the main servo motor ismultiplied by the position gain of the auxiliary servo motor.

FIG. 8 shows a state at the chuck-off time and a state at the chuck-ontime, and shows that only the switch SW3 switches from on-state tooff-state when chuck-on is made.

For that reason, if the states of the respective switches SW are set inthis way, the respective structures function as shown in FIGS. 3 and 9,and there is obtained the ratio in the position droop between the mainservo motor 14 and the auxiliary servo motor 24 (the value of theauxiliary servo motor is a denominator) which is obtained while the mainservo motor 14 and the auxiliary servo motor 24 are decoupled from eachother through the workpiece 1 and drive. When the main spindle 10 andthe auxiliary spindle 20 are coupled to each other through the workpiece1, and the main servo motor 14 and the auxiliary servo motor 24 rotate,the total position gain (position gain×change gear ratio) of theauxiliary servo motor 24 can be multiplied by the ratio of the positiondroop of the auxiliary servo motor to the position droop of the mainservo motor.

FIG. 9 shows the detailed correction processing (2) where the ratio ofthe position droop of the auxiliary servo motor to the position droop ofthe main servo motor is multiplied by the change gear ratio of theauxiliary servo motor.

In this processing, both of the main and auxiliary servo motors 14 and24 turn on the position control, and first of all, the main servo motor14 arithmetically operates the position droop Dr1 (=θ1*−θ1) of the mainspindle 10 (step 200), and the auxiliary servo amplifier 26arithmetically operates the position droop Dr2 (=θ2*−θ2) of theauxiliary spindle 20 (step 210).

In this situation, the switches SW3 and SW4 a are on, and all of otherswitches are off.

Then, the divider 9 a obtains the ratio γ of the position droops of boththe spindles which are obtained in the previous steps 200 and 210(γ=Dr2/Dr1) (step 220). Further, the ratio γ is stored in the memory 18a through the switch Sw3 (step 230).

Subsequently, it is judged whether the chucks 11 and 21 of both thespindles 10 and 20 turn on and both the spindles 11 and 21 are connectedto each other through the workpiece 1, or not (step 240). If the chucks11 and 21 are off (“no” in step 240), the switch Sw3 is kept on, and thestored ratio γ is further given to the change gear ratio multiplier 29through the change gear ratio correction section 30 including afirst-order lag processing filter therein to correct the change gearratio (step 250). Thereafter, the operation returns to the step 200, andthe above operation is repeated until the chucks turn on.

On the other hand, if the chucks 11 and 21 are on (“yes” in step 240),since the switch SW3 is turned off and storage of data in the memory 18a through the switch SW3 is interrupted from the next time, the ratio γis a fixed value (step 260). Thereafter, the operation returns to thestep 200, and the above operation is repeated until the chucks turn off.

That is, the third kind of correcting method shown in the correctionprocessing (2) does not require the change during speed changing ifcorrection is completed at once, but requires an increase in the numberof significant figures of the related memory since the change gear ratiois slight to the degree of slightly less than 1% even though it isinaccurate, with the result that the costs increase as much. Thus, thecorrection processing (2) has an advantage and a disadvantage which arenot obtained in the correction processing (1).

However, in case of the third kind of correcting method in thecorrection processing (2), the divider 9 a arithmetically operates theratio of the position droop of the main servo motor 14 and the positiondroop of the auxiliary servo motor 24 (the data of the auxiliary servomotor is a denominator), and the ratio of the position droop istransmitted to the change gear ratio correction section 30 through thememory 18 a. The change gear ratio correction section 30 subjects theratio of the position droops inputted by the filter built in the changegear ratio correction section 30 to a first-order lag processing toprevent a transitional fluctuation, produces a product of the changegear ratio stored in the change gear ratio multiplier 29 and thetransmitted position droop ratio, obtains the corrected change gearratio and gives it to the change gear ratio multiplier 29. The changegear ratio multiplier 29 multiplies the change gear ratio newlycorrected by the output from the position gain multiplier 28 and outputsthe multiplied value as a speed command.

The position gain correction section 30 a may be provided instead of theabove change gear ratio correction section 30. In other words, theposition gain correction section 30 a subjects the ratio of the positiondroops inputted by the filter built in the position gain correctionsection 30 a to a first-order lag processing to prevent a transitionalfluctuation, produces a product of the position gain within the positiongain multiplier 28 and the transmitted position droop ratio, obtains thecorrected position gain and gives it to the position gain multiplier 28.The position gain multiplier 28 multiplies the position gain newlycorrected by the output from the adder 38 a and outputs the multipliedvalue to the change gear ratio multiplier 29.

Therefore, in the correction processing (2) of the synchronizationcontrol device in accordance with the embodiment 1, since the positioncorrection is made in the feed-forward manner, after the main servomotor 14 and the auxiliary servo motor 24 are coupled to each otherthrough the workpiece, accurate synchronization drive is enabled even ifrapid acceleration and rapid deceleration is made.

(3) A processing where a value obtained by dividing the value of thefinal droop difference by the final speed in item (1) is stored by apredetermined r.p.m. or higher.

The conditions under which data are stored in the memory 18 or thememory 18 a in FIG. 2 or 3 in the above description, and the conditionsunder which data are stored in the memory in the step 140 of FIG. 7 orin the step 230 of FIG. 9 are that storage is executed by a given r.p.m.or higher.

This causes such problems that if the r.p.m. is too low, a largevariation occurs in the divided result of the divider 9 a due to aslight fluctuation of the position droop difference, and if the r.p.m.approaches to 0, the value of the divided result is excessive accordingto circumstances and overflow occurs in a digital processing.

Therefore, the r.p.m. is determined so that the divided result isstabilized, and data are stored with that r.p.m. or the higher as theconditions.

(4) A correction processing where a difference value in the droopdifference between the main servo motor and the auxiliary servo motor atthe time of chuck-on and chuck-off is added to the position command ofthe auxiliary servo motor.

FIG. 10 shows a state of the respective switches SW in the case ofconducting a correction processing where a difference value in the droopdifference between the main servo motor and the auxiliary servo motor atthe time of chuck-on and chuck-off is added to the position command ofthe auxiliary servo motor.

(a) shows a state of the respective switches SW when obtaining theposition droop reference value at the time of chuck-off, and (b) shows astate of the respective switches SW when further correcting a differencebetween the droop reference value at the time of chuck-on and theposition droop reference value at the time of chuck-off. In case of thecorrection processing (4), only the structures shown in FIGS. 11 and 4are required as in the above correction processing (1).

FIG. 11 shows a correction processing (4) where a difference value inthe position droop difference between the main servo motor and theauxiliary servo motor at the time of chuck-on and chuck-off is added tothe position command of the auxiliary servo motor.

In the processing, the output signal β1 of the comparator 7 that obtainsthe position droop difference between the main servo amplifier and theauxiliary servo amplifier is inputted after the processing of theflowchart shown in FIG. 7 (step 300), and the output signal β2 of themultiplier 19 that obtains a product of the present rotating speed isinputted to the memory 18 through the SW1-b (step 310). In thissituation, the switch SW5 b is kept on. Then, a difference β3 between β1and β2 (=β1−β2) is arithmetically operated in the subtractor 42 (step320).

Subsequently, it is judged whether the chucks 11 and 21 of both thespindles 10 and 20 turn on and both the spindles 11 and 21 are connectedto each other through the workpiece 1, or not (step 330). If the chucks11 and 21 are off (“no” in step 330), the switch SW6 a is turned off,and the connection of the output signal β3 of the subtractor 42 to asucceeding block is interrupted (step 360). Thereafter, the operationreturns to the initial operation and the above operation is repeateduntil the chucks turn on.

On the other hand, if the chucks 11 and 21 are on (“yes” in step 330),since the switch SW6 a is turned on and the output signal β3 is storedin the memory 37 through the switches SW5 b and SW6 a (step 340). Then,the switch SW6 a is turned off (step 350). Therefore, the value in thememory 37 becomes a fixed value until the succeeding chucks turn onbecause the switch SW6 a turns on at once.

Then, the output signal β3 stored in the memory 37 is inputted to theadder 38 through the filter 41 (added to the position command of theauxiliary servo motor) (step 370). Thereafter, the operation returns tothe initial operation, and the above operation is repeated until thechucks turn on again.

With the above operation, a change (a value after coupling—a valuebefore coupling) in a difference in the position droop between the mainservo motor and the auxiliary servo motor (the position droop of themain servo motor and the position droop of the auxiliary servo motor)immediately after the main spindle and the auxiliary spindle are coupledto each other through the workpiece, before coupling and after coupling,can be added to the position command of the auxiliary servo motoraccording to the states of the respective switches SW.

Therefore, in the processing (4) of the synchronization control devicein accordance with the embodiment 1, in the case where the main andauxiliary spindles are coupled to each other through the workpiece in astate where the servo motors are driven to conduct synchronizationcontrol, coupling of the spindles 10 and 20 to each other in a displacedstate can be prevented although the coupling is experientiallyfrequently made in the displaced state as a result of coupling in astate where a displacement occurs during coupling work even if the servomotors are completely synchronized with each other before coupling.

(5) A processing of selecting a function of setting a correction addedto the position command of the auxiliary spindle servo motor to 0immediately after coupling, and a function of accumulating adisplacement produced in each coupling to correct the position commandof the auxiliary spindle servo motor without returning the position to aposition immediately before coupling, when the two main and auxiliaryservo motors which have been coupled to each other through the workpieceare decoupled from each other.

The case of the correction processing (5) requires only the structureshown in FIG. 5 as in the above correction processing (1).

FIG. 12 shows a state of the respective switches SW in the case ofconducting a processing of selecting a function of setting a correctionadded to the position command of the auxiliary spindle servo motor to 0immediately after coupling, and a function of accumulating adisplacement produced in each coupling to correct the position commandof the auxiliary spindle servo motor without returning the position to aposition immediately before coupling.

Referring to FIG. 12, (a) shows a state of the respective switches SW atthe time of chuck-on in the case where this processing is conducted, (b)shows a state of the respective switches SW at the time of chuck-off inthe case where this processing is conducted, and (c) shows a state ofthe respective switches SW at the time of accumulating the displacementwithout returning correction at the time of chuck-off in the case wherethis processing is conducted. Selection can be made from (a) to (b) orfrom (a) to (c).

With the switching of the respective switches SW in the above state,according to the synchronization control device of this embodiment, whenthe servo motors which have been coupled to each other through theworkpiece are decoupled from each other, a function of returning acorrection added to the position command of the auxiliary servo motor tozero immediately after coupling, and a function of correcting theposition command of the auxiliary servo motor with the accumulated valueof the displacements produced every time coupling is made as a correctedvalue at the time of coupling without correcting the position command atthe time of decoupling.

The more details will be described. As shown in FIGS. 5 and 12, thecoupling-time displacement correction section 35 shown in FIG. 5arithmetically operates data obtained by subtracting a value based onthe inaccuracy of the change gear ratio from the difference in theposition droop between both the servo motors, and the switches SW6 a andSW6 c are switches that turn on only once immediately after coupling iscompleted (read data) and read zero when coupling is released by theswitches SW5 a and SW6 a/c. Also, the read data are stored in the memory37 and transmitted to the adder 38 through the filter 41 to correct theposition command.

FIG. 13 shows the detailed processing (5) of selecting a function ofsetting a correction added to the position command of the auxiliaryspindle servo motor to 0 immediately after coupling, and a function ofaccumulating a displacement produced in each coupling to correct theposition command of the auxiliary spindle servo motor without returningthe position to a position immediately before coupling, when the mainand auxiliary servo motors which have been coupled to each other throughthe workpiece are decoupled from each other.

In this processing, it is judged whether or not the chuck 21 of theauxiliary spindle 20 is turned on and both the spindles 10 and 20 areconnected to each other through the workpiece 1 after the processing ofthe flowcharts shown in FIGS. 7 and 11 in a state where the chuck 11 ofthe main spindle 10 is turned on and nips the workpiece (step 400). Ifthe chuck 21 is off (“no” in step 400), the operation jumps up to step440. If the chuck 21 is on (“yes” in step 400), the positiondisplacement correcting method at the time of coupling through theworkpiece is judged (step 410).

If it is not of the accumulated type (the chuck 21 of the auxiliaryspindle is turned off (workpiece coupling is decoupled) and a correctedvalue is arithmetically calculated again and stored at the time ofturning on the chuck 21 again) (“no” in step 410), the operation jumpsup to the step 440. On the other hand, if it is of the accumulated type(the chuck 21 of the auxiliary spindle is turned off (workpiece couplingis decoupled) and the present corrected value is accumulated in theprevious corrected value and stored at the time of turning on the chuck21 again) (“yes” in step 410), the switches SW6 b and SW6 c are turnedon, and “previous value+present corrected value” is stored in theaccumulating memory 44 and also stored in the memory 37 through theswitch SW6 c. In this situation, the switch states are that SW5 a isoff, SW5 b is on, and SW6 a is off (step 420). Then, the switches SW6 band SW6 c are turned off (step 430).

Then, it is judged whether the chuck 21 of the auxiliary spindle 20 isturned off and both the spindles 10 and 20 are connected to each otherthrough the workpiece 1, or not (step 440). If the chuck 21 is on (“no”in step 440), the operation returns to the initial operation and theabove operation is repeated. If the chuck 21 is off (“yes” in step 440),it is judged whether the position displacement corrected value at thetime of decoupling is canceled, or not (step 450). If cancel is not made(“no” in step 450), the operation returns to the initial operation andthe above operation is repeated. If cancel is made (“yes” in step 450),the switch SW5 a is turned on, the SW5 b is turned off and the SW6 a isturned on to clear the memory 37 to zero (step 460). Then, the switchSW5 a is turned off, the SW5 b is turned on and the SW6 a is turned off,thus returning the respective switches to the original states (step470). Thereafter, the operation returns to the initial operation and theabove operation is repeated.

In other words, since this correction keeps an electric balance bydisplacing a reference point by a displacement produced when coupling, aphase difference occurs between both the spindles when the mechanicalcoupling is released. In general, it is preferable that the position isreturned to an initial positional relation. If returning is made, thevalue in the memory is set to 0. If the displaced reference point is setas a new reference without returning the displacement produced at thetime of coupling to the original position, the displacement produced ineach coupling is accumulated and used as the corrected value. The switchSW6 is a switch that enables the selection of the functions for dealingwith the displacement produced at the time of coupling. Because theposition command in this description is of the absolute position system,the corrected data are merely added. However, if the position command isof the incremental value system, addition is ended at the time when theaccumulation of the added position commands coincides with the correcteddata.

Therefore, in the correction processing (4) of the synchronizationcontrol device in accordance with the embodiment 1, if the positioncommand is corrected at the time of coupling, the position of thespindle is displaced by the corrected amount when coupling is released.However, the displacement can be returned to 0 according to theapplication or can be accumulated with the displaced position as areference.

(6) A processing that can change the speed control to the positioncontrol and vice versa when the speed command is within a predeterminederror.

FIG. 14 shows a state of the respective switches SW in the case ofconducting a processing that can change the speed control to theposition control and vice versa when the speed command is within apredetermined error.

Referring to FIG. 14, (a) shows a state of the respective switches SW incase of a speed control loop, and (b) shows a state of the respectiveswitches SW in case of a position control loop. The switch SW4 isswitched from an off-state (X) to an on-state (O) under the control bythe comparator 36 even in the state of the position control loop (b).

In other words, assuming that operation is made in response to the speedcommand, in the switch state shown in FIG. 14, the main and auxiliaryservo amplifiers 16 and 26 produce position droop signals which come tothe speed commands corresponding to the speed of the spindles on thebasis of the position information from the position detectors 15 and 25of the main and auxiliary spindles 10 and 20.

A description will be given with reference to FIG. 2. The comparator 36compares the output signal of the position gain multiplier 28 of thechange gear ratio that constitutes the position control loop, that is,the change gear ratio with the servo motor 24 side as a numerator andthe change gear ratio multiplier 29 with the speed command, and if adifference therebetween is within a permitted value, the control isswitched to the position control by the switch SW4. On the other hand,if the difference therebetween is not within the permitted value, thecontrol is switched from the position control to the speed control bythe switch SW4.

For that reason, since the respective switches SW are switched in theabove state, the position droop is added to the position detectionsignal of a machine driven by the servo motors to produce the positioncommand, a difference between the speed command to the servo motorswhich is produced from that position command and the speed commandduring operation under the speed control is within a predeterminederror, the control system of the servo motors can be switched from thespeed control to the position control.

FIG. 15 shows the speeds of the main servo motor and the auxiliary servomotor and a switching state between the speed control and the positioncontrol due to the correction processing (6), in which the positioncontrol is made before and after chuck-on including at leaston-chuck-on, and the speed control is made before and after thesynchronization on and before and after the synchronization off.

Therefore, in the correction processing (6) of the synchronizationcontrol device in accordance with the embodiment 1, when the servo motorof the position control and the servo motor of the speed control arecompared with each other in acceleration and decelerationcharacteristics, the acceleration/deceleration can be made by all thetorque produced by the servo motors in the speed control, whereas allthe torque produced cannot be used as the acceleration/decelerationtorque with the limit of the acceleration/deceleration characteristicsin the position control because the acceleration/decelerationcharacteristics influence the position. However, when the r.p.m. risesfrom an initial stop to a rated r.p.m., it rises under the speed controland after it reaches the rated r.p.m., the control is switched from thespeed control to the position control, thereby being capable ofshortening the acceleration period of time.

The above description of the embodiment 1, is made on a machining devicehaving two spindles consisting of one main spindle 10 and an auxiliaryspindle 20, respectively. However, if two or more spindles are provided,the main servo motor is fitted to one spindle, one of plural auxiliaryservo motors and one main servo motor are coupled to each other whileobserving the position droops of both the servo motors, and thereafteranother auxiliary servo motor and the main servo motor are coupled toeach other. In this way, if the auxiliary servo motors are sequentiallycoupled to the main servo motor, it is not necessary to conduct thesynchronization control of the main servo motor with one auxiliary servomotor with a limit at all.

Also, in the embodiment 1, as described above, the structure within theauxiliary servo amplifier 26 is improved without changing the structurewithin the main servo amplifier 16. Conversely, it is needless to saythat the structure within the main servo amplifier 16 may be improved asdescribed above, and the structure within the auxiliary servo amplifier26 may not be changed.

As described above, according to the present invention, in thesynchronization control device using a plurality of servo motors whichcan be mechanically coupled or decoupled through the workpiece, therespective position droops between one main servo motor and otherauxiliary servo motors are compared with each other, and if the mainservo motor and the auxiliary servo motor are not mechanically coupledto each other, a difference in the position droop with respect to themain servo motor is added to the position command of the auxiliary servomotor to correct the position. If the main servo motor and the auxiliaryservo motor are mechanically coupled to each other through theworkpiece, a value obtained by dividing a difference in the positiondroops between both the servo motors which are obtained in the operationbefore they are coupled to each other through the workpiece (the mainservo motor value−the auxiliary servo motor value) by the speed of theauxiliary servo motor at the time of measurement is stored. If the mainservo motor and the auxiliary servo motor are coupled to each otherthrough the workpiece, a product value of a difference of the positiondroop per the unit speed and the speed of the servo motor is added tothe position command of the auxiliary servo motor.

With the above operation, even if a plurality of servo motors arecoupled to each other through the workpiece by means of a transmissionmechanism of a flexible structure such as belting and a rapidacceleration or deceleration drive is made in synchronization in thatstate without changing the corrected value every time the speed of theservo motors changes, the torque of the servo motors can be realizedwithout any loss caused by synchronization drive.

In other words, according to the present invention, a difference inposition between the servo motors which are coupled to each otherthrough the workpiece is grasped before mechanical coupling, and aftercoupling, control is made without any lag in a feed-forward manner, thatis, in a manner that the data are installed in arithmetic operation inadvance on the basis of the data grasped before coupling. Accordingly,even if the change gear ratio is not accurately determined due to theuse of a belt as the transmission mechanism of a mechanical sectionwhich is driven by the servo motors, accurate synchronization drive isenabled, rapid acceleration and deceleration are enabled in themechanical coupling state, and complete synchronization control can beconducted even in the acceleration/deceleration state.

As a result, in case of belting, since it is supposed that the belt isdeteriorated with a time unlike to a gear, etc., if the r.p.m. is not solow in the state where the servo motors are decoupled from each otherthrough the workpiece, data are updated and data to be used aftercoupling are prepared so as to cope with a deterioration with a time.

Also, the change gear ratio is obtained from the respective positiondroops of the main servo motor and the auxiliary servo motor and thedifference in the droop between both the main and auxiliary servo motorsis corrected by use of the change gear ratio. Therefore, in this case,although the number of places of decimals increases because of a normaldifference within 1% and a memory of a large capacity is required, ifaccurate setting is made once, no correction is required even if thespeed changes.

In other words, in the correcting method in the feed-forward manner, thedifference in position is added to the position command for correction.In the method, the corrected value is required to be corrected inproportion to the speed, but if the total position gain which is aproduct of the position gain and the change gear ratio is corrected, thecorrected value per se may not be changed while it is correct afterchanging. However, since the number of significant figures of decimalsmust be greatly increased in order to represent a slight error, thismethod needs to be selected according to circumstances. Since thepresent invention is directed to control in the case where the changegear ratio of the transmission mechanism is not accurately represented,the position control loop is a command to an object to be driven such asa spindle, the feedback signal is directly obtained from the movement ofa machine, and the position gain constitutes the position control loopresulting from multiplying the position gain responsive to an end of themachine by the change gear ratio of the transmission mechanism.

Further, according to the present invention, the ratio in the positiondroop between both the main servo motor and the auxiliary servo motorwhich is obtained while the main servo motor and the auxiliary servomotor are decoupled from each other through the workpiece and drive (thevalue of the auxiliary servo motor is a denominator) is obtained, andwhen the main servo motor and the auxiliary servo motor are mechanicallycoupled to each other through the workpiece, the total position gain ofthe auxiliary servo motor (position gain×change gear ratio) ismultiplied by the ratio of the above position-droops.

With the above operation, although it is supposed that what transmits apower with the flexible structure such as the belt of this type isdeteriorated in constant with a time, since data used for correction arecontinuously updated to new data in the present invention, thecorrection can cope with a deterioration with a time. Also, since theposition correction is made in a feed-forward manner, accuratesynchronization drive can be made even in rapid acceleration or rapiddeceleration after the main servo motor and the auxiliary servo motorare coupled to each other through the workpiece.

Still further, according to the present invention, a change (a valueafter coupling−a value before coupling) in a difference in the positiondroop between the main servo motor and the auxiliary servo motor (theposition droop of the main servo motor and the position droop of theauxiliary servo motor) immediately after the main spindle and theauxiliary spindle are mechanically coupled to each other through theworkpiece, before coupling and after coupling, can be added to theposition command of the auxiliary servo motor.

With the above operation, a mechanical displacement which is liable tooccur when coupling is made through the workpiece can be electricallytreated without any problems. In other words, in the case where the mainspindle and the auxiliary spindle are coupled to each other through theworkpiece to conduct synchronization control in a state where the mainand auxiliary servo motors are driven, the coupling is experientiallyfrequently made in the displaced state as a result of coupling in astate where a displacement occurs during coupling work even if the servomotors are completely synchronized with each other before coupling.However, the displacement can be compensated without any problems evenif such a mechanical displacement occurs.

Yet still further, according to the present invention, it is possible toselect a function of setting a correction added to the position commandof the auxiliary spindle servo motor to 0 immediately after coupling,and a function of correcting the position command of the auxiliaryspindle servo motor to the accumulated value of displacements producedin each coupling as a corrected value at the time of coupling withoutcorrecting the position command at the time of decoupling, when the mainand auxiliary servo motors which have been coupled to each other throughthe workpiece are decoupled from each other.

With the above operation, when the mechanical coupling through theworkpiece is released, even if the position of the spindle is displacedby the corrected amount, the machine position can be returned to theoriginal state with elimination of the displacement or an advancingfunction can be selected while the displacement state is set as areference according to the application.

In other words, according to the present invention, when the mainspindle and the auxiliary spindle are decoupled from each other, thedisplacement produced during coupling work is corrected to the originalor the subsequent displacements are accumulated with the displacedposition as a new reference according to the applied method. Since anyof the above methods can be selected in the synchronization controlaccording to the present invention, not only the inaccuracy of thechange gear ratio of the transmission machine such as a belt can besolved, but also a correction for eliminating saturation of the torquecaused by coupling with a temporally positional displacement duringcoupling work can be made. Thus, another problem to a quick response isalso solved.

Yet still further, according to the present invention, the positiondroop is added to the position detection signal of a machine driven bythe servo motors to produce the position command, and when a differencebetween the speed command to the servo motors which is produced fromthat position command and the speed command during operation under thespeed control is within a predetermined error, the control system of theservo motors can be switched from the speed control to the positioncontrol.

With the above operation, when the r.p.m. rises from an initial stop toa rated r.p.m., it rises under the speed control and after it reachesthe rated r.p.m., the control is switched from the speed control to theposition control, thereby being capable of shortening the accelerationperiod of time.

In other words, when the servo motor of the position control and theservo motor of the speed control are compared with each other inacceleration and deceleration characteristics, theacceleration/deceleration can be made by all the torque produced by theservo motors in the speed control, whereas all the torque producedcannot be used as the acceleration/deceleration torque with the limit ofthe acceleration/deceleration characteristics in the position controlbecause the acceleration/deceleration characteristics influence theposition. Although the present invention is mainly directed to theposition control, the torque which can be produced by the servo motorscan be utilized at the maximum if the speed control is appropriatelyused.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the positiondroops produced in the main and auxiliary servo motors when the main andauxiliary spindles are coupled to each other through the workpiece areobtained by the adders, respectively, a difference between thoseposition droops is obtained by the comparator, the divider divides theposition droop difference by the speed of the auxiliary servo motor atthe time of detecting the position droop to obtain a position droopdifference per a unit speed. Then, after both the spindles are coupledto each other, the multiplier multiplies the position droop differenceby the present speed of the auxiliary servo motor, and the adder addsthat value to the position command to the auxiliary servo motor. Withthe above structure, even if coupling is made through the workpiece andsynchronization drive is made, an excessive torque can be prevented fromoccurring, and even if a difference occurs in the position droop betweenboth the spindles when the main spindle and the auxiliary spindle arecoupled to each other, accurate synchronization drive of the mainspindle and the auxiliary spindle is enabled, and rapid acceleration ordeceleration drive is enabled. In particular, even if a flexiblestructure such as a belt is used for a torque transmission between themain spindle and the servo motor, the synchronization control devicewhich can automatically cope with a change of the change gear ratio witha time can be provided.

What is claimed is:
 1. A synchronization control device for a servomotor comprising: a main spindle and an auxiliary spindle each having achuck that holds both ends of a workpiece and being coupled to ordecoupled from each other through said workpiece; a main servo motor andan auxiliary servo motor which rotationally drive said main spindle andsaid auxiliary spindle, respectively; a main spindle position detectorand an auxiliary spindle position detector which are fitted to said mainspindle and said auxiliary spindle, respectively, and output a positionfeedback signal; a main servo motor speed detector and an auxiliaryservo motor speed detector which are fitted to said main servo motor andsaid auxiliary servo motor, respectively, and output a speed feedbacksignal; a main servo amplifier and an auxiliary servo amplifier whichdrive said main servo motor and said auxiliary servo motor,respectively; and a numerical control unit which outputs a speed commandand a position command to said main servo amplifier and said auxiliaryservo amplifier, wherein each of said main servo amplifier and saidauxiliary servo amplifier includes: a position control section whichoutputs a position droop and the speed command responsive to theposition droop on the basis of inputs of the position feedback signalfrom said position detector and the position command from said numericalcontrol unit; a speed control section which outputs a current command onthe basis of the speed command from said position control section andthe speed feedback signal from said speed detector; and a currentcontrol section which controls a supply current to the motor on thebasis of the current command from said speed control section and thecurrent feedback signal which flows in the motor, wherein said auxiliaryservo amplifier further includes a position correction section whichobtains position correction data based on a difference in position droopbetween the main spindle position droop and the auxiliary spindleposition droop and based on the speed feedback signal from the speeddetector of said auxiliary servo motor when said main servo motor andsaid auxiliary servo motor rotate in a state where said main spindle andsaid auxiliary spindle are decoupled from each other, and wherein theposition control section of said auxiliary servo amplifier adds theposition correction data from said position correction section to theposition droop and outputs the speed command responsive to the positiondroop obtained by addition when said main spindle and said auxiliaryspindle are coupled to each other through said workpiece, and said mainservo motor and said auxiliary servo motor rotate in synchronizationwith each other.
 2. The synchronization control device for a servo motoras claimed in claim 1, characterized in that said position correctionsection includes: comparing means for obtaining a position droopdifference between the main spindle position droop and the auxiliaryspindle position droop when said main servo motor and said auxiliaryservo motor rotate in a state where said main spindle and said auxiliaryspindle are decoupled from each other; dividing means for dividing theposition droop difference from said comparing means by the speedfeedback signal from said speed detector of said auxiliary servo motor;memory means for storing a division value from said dividing means,multiplying means for multiplying the division value stored in saidmemory means by the speed feedback signal from said speed detector ofsaid auxiliary servo motor; and switch means for switching so as tooutput, as the position correction data which are outputted to saidposition control section of said auxiliary servo amplifier, the positiondroop difference from said comparing means when said main servo motorand said auxiliary servo motor rotate in a state where said main spindleand said auxiliary spindle are decoupled from each other, and to output,as the position correction data which are outputted to said positioncontrol section of said auxiliary servo amplifier, a multiplicationvalue from said multiplying means when said main spindle and saidauxiliary spindle are coupled to each other through said workpiece, andsaid main servo motor and said auxiliary servo motor rotate.
 3. Thesynchronization control device for a servo motor as claimed in claim 1,characterized in that said position correction section includes:position droop ratio calculating means for calculating the ratio of theposition droop of said main spindle to the position droop of saidauxiliary spindle when said main servo motor and said auxiliary servomotor rotate in a state where said main spindle and said auxiliaryspindle are decoupled from each other; and memory means for storing theposition droop ratio from said position droop ratio calculating means;wherein said position control section includes: position droopcalculating means for calculating the position droop of said auxiliaryspindle on the basis of inputs of the position feedback signal from saidposition detector and the position command from said numerical controlunit; position gain multiplying means for multiplying the position droopfrom said position droop calculating means by a position gain of saidauxiliary servo motor which is stored in advance; change gear ratiomultiplying means for multiplying a multiplication output from saidposition gain multiplying means by a change gear ratio of said auxiliaryservo motor which is stored in advance; and change gear ratio correctingmeans for correcting the change gear ratio by multiplying the changegear ratio which is stored in said change gear ratio multiplying meansin advance by the position droop ratio which is stored in said memorymeans when said main spindle and said auxiliary spindle are coupled toeach other through said workpiece, and said main servo motor and saidauxiliary servo motor rotate in synchronization with each other.
 4. Thesynchronization control device for a servo motor as claimed in claim 3,characterized in that said change gear ratio correcting means includes afilter which subjects the position droop ratio stored in said memorymeans to a first-order lag processing.
 5. The synchronization controldevice for a servo motor as claimed in claim 1, characterized in thatsaid position correction section includes: position droop ratiocalculating means for calculating the ratio of the position droop ofsaid main spindle to the position droop of said auxiliary spindle whensaid main servo motor and said auxiliary servo motor rotate in a statewhere said main spindle and said auxiliary spindle are decoupled fromeach other; and memory means for storing the position droop ratio fromsaid position droop ratio calculating means; wherein said positioncontrol section includes: position droop calculating means forcalculating the position droop of said auxiliary spindle on the basis ofinputs of the position feedback signal from said position detector andthe position command from said numerical control unit; position gainmultiplying means for multiplying the position droop from said positiondroop calculating means by a position gain of said auxiliary servo motorwhich is stored in advance; change gear ratio multiplying means formultiplying a multiplication output from said position gain multiplyingmeans by a change gear ratio of said auxiliary servo motor which isstored in advance; and position gain correcting means for correcting theposition gain by multiplying the position gain which is stored in saidposition gain multiplying means in advance by the position droop ratiowhich is stored in said memory means when said main spindle and saidauxiliary spindle are coupled to each other through said workpiece, andsaid main servo motor and said auxiliary servo motor rotate insynchronization with each other.
 6. The synchronization control devicefor a servo motor as claimed in claim 5, characterized in that saidposition gain correcting means includes a filter which subjects theposition droop ratio stored in said memory means to a first-order lagprocessing.
 7. The synchronization control device for a servo motor asclaimed in claim 2, characterized in that said position correctionsection stores, in said memory means, a division value obtained fromdividing the position droop difference from said comparing means by thespeed feedback signal from the speed detector of said auxiliary servomotor when said main servo motor and said auxiliary servo motor rotateby a predetermined r.p.m. or higher in a state where said main spindleand said auxiliary spindle are decoupled from each other.
 8. Thesynchronization control device for a servo motor as claimed in claim 3,characterized in that said position correction section stores in saidmemory means the position droop ratio from said position droop ratiocalculating means when said main servo motor and said auxiliary servomotor rotate by a predetermined r.p.m. or higher in a state where saidmain spindle and said auxiliary spindle are decoupled from each other.9. The synchronization control device for a servo motor as claimed inclaim 2, characterized in that said position correction section furtherincludes: a switch disposed between said multiplying means and saidstoring means, said switch being closed when said main servo motor andsaid auxiliary servo motor rotate in a state where said main spindle andsaid auxiliary spindle are decoupled from each other, said switch beingopened when said main spindle and said auxiliary spindle are coupled toeach other through said workpiece, and said main servo motor and saidauxiliary servo motor rotate; a subtractor which subtracts, from theposition droop from said comparing means, the position droop due to themultiplication value from said multiplying means that multiplies thedivision value stored in said memory means through said switch by thespeed feedback signal from said speed detector of said auxiliary servomotor when said main spindle and said auxiliary spindle are coupled toeach other through said workpiece, and said main servo motor and saidauxiliary servo motor rotate; and a coupling-time displacementcorrection section which outputs a difference, in the position droopbetween said main spindle and said auxiliary spindle which is obtainedfrom said subtractor and produced before and after said main spindle andsaid auxiliary spindle are coupled to each other through said workpiece,to said position control section as the position correction data. 10.The synchronization control device for a servo motor as claimed in claim9, characterized in that said coupling-time displacement correctionsection further includes: zero data generating means for generating zerodata as the position correction data; and accumulating means foraccumulating the position correction data from said subtractor which areproduced every time said main spindle and said auxiliary spindle arecoupled to each other through the workpiece, wherein when saidcoupling-time displacement correction section outputs the zero data fromsaid zero data generating means or the position correction data fromsaid accumulating means as the position correction data which areoutputted to said position control section immediately after coupling,when said main spindle and said auxiliary spindle which have beencoupled to each other through said workpiece, are decoupled from eachother.
 11. The synchronization control device for a servo motor asclaimed in claim 1, further comprising: a comparator which compares thespeed command from said position control section with the speed commandfrom said numerical control section; and command switching means foroutputting the speed command from said position control section to saidspeed control section in order to conduct position control when acomparison difference is within a given error, and for outputting thespeed command from said numerical control section to said speed controlsection in order to conduct position control when the comparisondifference is not within the given error.
 12. The synchronizationcontrol device for a servo motor as claimed in claim 1, wherein saidposition correction section comprises: a comparator operative to obtaina position droop difference between the main spindle position droop andthe auxiliary spindle position droop when said main servo motor and saidauxiliary servo motor rotate in a state where said main spindle and saidauxiliary spindle are decoupled from each other; a divider operative todivide the position droop difference from said comparator by the speedfeedback signal from said speed detector of said auxiliary servo motor;a memory operative to store a division value from said divider, amultiplier operative to multiply the division value stored in saidmemory by the speed feedback signal from said speed detector of saidauxiliary servo motor; and a switch operative to switch so as to output,as the position correction data which are outputted to said positioncontrol section of said auxiliary servo amplifier, the position droopdifference from said comparator when said main servo motor and saidauxiliary servo motor rotate in a state where said main spindle and saidauxiliary spindle are decoupled from each other, and to output, as theposition correction data which are outputted to said position controlsection of said auxiliary servo amplifier, a multiplication value fromsaid multiplier when said main spindle and said auxiliary spindle arecoupled to each other through said workpiece, and said main servo motorand said auxiliary servo motor rotate.
 13. The synchronization controldevice for a servo motor as claimed in claim 1, wherein said positioncorrection section comprises: a position droop ratio calculatoroperative to calculate the ratio of the position droop of said mainspindle to the position droop of said auxiliary spindle when said mainservo motor and said auxiliary servo motor rotate in a state where saidmain spindle and said auxiliary spindle are decoupled from each other;and a memory operative to store the position droop ratio from saidposition droop ratio calculator; and wherein said position controlsection includes: a position droop calculator operative to calculate theposition droop of said auxiliary spindle on the basis of inputs of theposition feedback signal from said position detector and the positioncommand from said numerical control unit; a position gain multiplieroperative to multiply the position droop from said position droopcalculator by a position gain of said auxiliary servo motor which isstored in advance; a change gear ratio multiplier operative to multiplya multiplication output from said position gain multiplier by a changegear ratio of said auxiliary servo motor which is stored in advance; anda change gear ratio corrector operative to correct the change gear ratioby multiplying the change gear ratio which is stored in said change gearratio multiplier in advance by the position droop ratio which is storedin said memory when said main spindle and said auxiliary spindle arecoupled to each other through said workpiece, and said main servo motorand said auxiliary servo motor rotate in synchronization with eachother.
 14. The synchronization control device for a servo motor asclaimed in claim 13, wherein said change gear ratio corrector includes afilter operative to subject the position droop ratio stored in saidmemory to a first-order lag processing.
 15. The synchronization controldevice for a servo motor as claimed in claim 1, wherein said positioncorrection section includes: a position droop ratio calculator operativeto calculate the ratio of the position droop of said main spindle to theposition droop of said auxiliary spindle when said main servo motor andsaid auxiliary servo motor rotate in a state where said main spindle andsaid auxiliary spindle are decoupled from each other; and a memoryoperative to store the position droop ratio from said position droopratio calculator; and wherein said position control section includes: aposition droop calculator operative to calculate the position droop ofsaid auxiliary spindle on the basis of inputs of the position feedbacksignal from said position detector and the position command from saidnumerical control unit; a position gain multiplier operative to multiplythe position droop from said position droop calculator by a positiongain of said auxiliary servo motor which is stored in advance; a changegear ratio multiplier operative to multiply a multiplication output fromsaid position gain multiplier by a change gear ratio of said auxiliaryservo motor which is stored in advance; and a position gain correctoroperative to correct the position gain by multiplying the position gainwhich is stored in said position gain multiplier in advance by theposition droop ratio which is stored in said memory when said mainspindle and said auxiliary spindle are coupled to each other throughsaid workpiece, and said main servo motor and said auxiliary servo motorrotate in synchronization with each other.
 16. The synchronizationcontrol device for a servo motor as claimed in claim 15, wherein saidposition gain corrector includes a filter which is operative to subjectthe position droop ratio stored in said memory to a first-order lagprocessing.
 17. The synchronization control device for a servo motor asclaimed in claim 12, wherein said position correction section isoperative to store, in said memory, a division value obtained fromdividing the position droop difference from said comparator by the speedfeedback signal from the speed detector of said auxiliary servo motorwhen said main servo motor and said auxiliary servo motor rotate by apredetermined r.p.m. or higher in a state where said main spindle andsaid auxiliary spindle are decoupled from each other.
 18. Thesynchronization control device for a servo motor as claimed in claim 13,wherein said position correction section is operative to store in saidmemory the position droop ratio from said position droop ratiocalculator when said main servo motor and said auxiliary servo motorrotate by a predetermined r.p.m. or higher in a state where said mainspindle and said auxiliary spindle are decoupled from each other. 19.The synchronization control device for a servo motor as claimed in claim12, wherein said position correction section further includes: a switchdisposed between said multiplier and said memory, said switch beingclosed when said main servo motor and said auxiliary servo motor rotatein a state where said main spindle and said auxiliary spindle aredecoupled from each other, said switch being opened when said mainspindle and said auxiliary spindle are coupled to each other throughsaid workpiece, and said main servo motor and said auxiliary servo motorrotate; a subtractor which subtracts, from the position droop from saidcomparator, the position droop due to the multiplication value from saidmultiplier that multiplies the division value stored in said memorythrough said switch by the speed feedback signal from said speeddetector of said auxiliary servo motor when said main spindle and saidauxiliary spindle are coupled to each other through said workpiece, andsaid main servo motor and said auxiliary servo motor rotate; and acoupling-time displacement correction section which outputs a differencein the position droop between said main spindle and said auxiliaryspindle which is obtained from said subtractor and produced before andafter said main spindle and said auxiliary spindle are coupled to eachother through said workpiece to said position control section as theposition correction data.
 20. The synchronization control device for aservo motor as claimed in claim 19, wherein said coupling-timedisplacement correction section further includes: a zero data generatoroperative to generate zero data as the position correction data; and anaccumulator operative to accumulate the position correction data fromsaid subtractor which are produced every time said main spindle and saidauxiliary spindle are coupled to each other through the workpiece; andwherein when said coupling-time displacement correction section outputsthe zero data from said zero data generator or the position correctiondata from said accumulator as the position correction data which areoutputted to said position control section immediately after couplingwhen said main spindle and said auxiliary spindle, which have beencoupled to each other through said workpiece, are decoupled from eachother.
 21. The synchronization control device for a servo motor asclaimed in claim 1, further comprising: a comparator which compares thespeed command from said position control section with the speed commandfrom said numerical control section; and a command switch operative tooutput the speed command from said position control section to saidspeed control section, in order to conduct position control when acomparison difference is within a given error, and operative to outputthe speed command from said numerical control section to said speedcontrol section, in order to conduct position control when thecomparison difference is not within the given error.